This invention relates to a lens driving device and, in particular, to leaf springs (a supporting member; an elastic member) which are disposed both sides of a tubular portion of a lens holder (a movable portion) for holding a lens assembly (a lens barrel) in the direction of an optical axis and which support the lens holder in the direction of the optical axis shiftably so as to position the lens holder in a radial direction with respect to a housing (a fixed portion).
The portable compact camera is mounted in a camera-equipped cellular mobile phone. The portable compact camera is provided with the autofocus lens driving device. Previously, various autofocus lens driving devices have been proposed.
As a driving source (a driving method) used in such a lens driving device, there are a VCM method using a voice coil motor (VCM) and a SMA method using shape memory alloy (SMA). The lens driving device of either of the above-mentioned two methods uses a supporting member (an elastic member) which supports a movable portion (a lens pulse a lens holder) including the lens in the direction of an optical axis (the direction of a center axis) shiftably so as to position the movable portion in a radial direction with respect to a fixed portion. As such as a supporting member (an elastic member), at least one leaf spring is generally used.
Conventionally, as a material of the leaf spring, beryllium copper or SUS base steel material is used (see, e.g. Japanese Unexamined Patent Application Publication No. 2009-122360 (JP-A 2009-122360)).
In addition, various plate springs having improved impact resistance have been proposed.
By way of illustration, Japanese Unexamined Patent Application Publication No. 2009-210055 (JP-A 2009-210055) discloses a cheap leaf spring having improved impact resistance. The leaf spring comprises an inner edge portion (an inner ring portion) and an outer edge portion (an outer ring portion) each of which is formed in an annular ring shape, and N arm portions disposed along a circumferential direction in order to couple them. Each arm portion extends along the circumferential direction. Disposed between the inner edge portion and each arm portion, an inner coupling portion protrudes from the inner edge portion in a radial direction outwardly. Disposed between the outer edge portions and each arm portion, an outer coupling portion protrudes from the outer edge portion in a radial direction inwardly. At least one of an inner basal portion and an outer basal portion of the arm portion has a plate width which is wider than that of a longitudinally center portion of the arm portion.
In addition, Japanese Unexamined Patent Application Publication No. 2007-322540 (JP-A 2007-322540) discloses a camera module superior in impact resistance by preventing stress concentration to a base part (a basal portion) of a crosslinking parts (arm portions) of a leaf spring due to impact. The leaf spring disclosed in JP-A 2007-322540 comprises an outer ring part, an inner ring part displacing to the outer ring part, and three crosslinking parts (arm portions) for supporting the inner ring part to the outer ring part. Each crosslinking part extends along the inner peripheral edge of the outer ring part and the outer peripheral edge of the inner ring part in a ring-like gap formed between the outer ring part and the inner ring part. The crosslinking part has a slit serving as stress concentration prevention means.
In the cellular mobile phone equipping the portable compact camera driven by the above-mentioned autofocus lens driving device, improvement (increase) of two kinds of performance (characteristic) is required as follows. A first kind of performance (characteristic) is impact resistant performance (characteristic) with respect to drop or like on use and storage of the cellular mobile phone. A second kind of performance (characteristic) is lens inclination (tilt) performance (characteristic) on operation with large pixels of the camera equipped in the cellular mobile phone.
The two kinds of performance (characteristic) has a dependence on the above-mentioned leaf spring largely. In order to improve (increase) the impact resistant performance (characteristic), it may make “distortion” of stiffness of the leaf spring in a plane direction small. On the other hand, in order to improve (increase) the lens inclination (tilt) performance (characteristic) on operation, it may make “torsional rigidity” of the whole (leaf spring pulse lens holder) large.
However, the stiffness of the leaf spring in the plane direction and the “torsional rigidity” of the whole (leaf spring pulse lens holder) are indirectly proportional to each other. That is, the impact resistant performance (characteristic) and the lens inclination (tilt) performance (characteristic) where the leaf spring is predominant are indirectly proportional to each other.
It is known in the art that beryllium copper used as the material of the leaf spring is superior in the above-mentioned first and second kinds of performance (characteristic). Therefore, beryllium copper is widely used as material of a high performance spring. However, it is also known in the art that a compound of beryllium is highly poisonous. It is therefore desirable that material other than beryllium copper is used as the material of the leaf spring in view of environment, namely, beryllium-free.
On the other hand, in the lens driving device adopting the VCM method as a driving method, permanent magnets are used as driving means. Therefore, if a magnetic material having high magnetic permeability is used as the material of the leaf spring, it has a detrimental effect on performance of the lens driving device. Accordingly, it is preferable that material having low magnetic permeability is used as the material of the leaf spring. Stainless steel used normally is non-magnetic material where relative magnetic permeability is about one.